Inverter question

[Theo]

Since it looks as if I am going to buy a new inverter I have been studying the data on the internet. Just one of the modified sine wave inverters that I looked at said that it would not run a fridge because of the problems with an inductive load. The other MSW units that I looked at did not mention any problems in that way. I wonder if it is a problem of damage to the fridge motor or damage to the inverter. Can I overcome the problem, if it exists, by derating?

 

I would be interested to hear your views.

 

Nick.

[NB Alnwick]

Since it looks as if I am going to buy a new inverter I have been studying the data on the internet. Just one of the modified sine wave inverters that I looked at said that it would not run a fridge because of the problems with an inductive load. The other MSW units that I looked at did not mention any problems in that way. I wonder if it is a problem of damage to the fridge motor or damage to the inverter. Can I overcome the problem, if it exists, by derating?

 

I would be interested to hear your views.

 

Nick.

 

We have an Electrolux fridge that we can switch between gas, low voltage direct current and 240v AC.

 

When we are not running the engine it runs on gas, when we are running the engine it runs off the dc supply and when we are hooked up to the mains it runs off 240v - easy . . .

 

Incidentally - it uses very little gas - a 13 Kg cylinder lasts us about four weeks and we also use the same gas for cooking and hot water for washing.

Edited January 11, 2007 by NB Alnwick

[Keeping Up]

I think it is most likely to be a problem of potential damage to the inverter, or else the system not working because of the inverter shutting down to protect itself. I would imagine that simply using a larger, pure sine wave inverter would get you round the problem.

 

Another reason that many inverters have in running a fridge, is that most 240v fridges have quite a high startup current every time the motor kicks in. It's not a big problem when they are installed in a house, but on a boat it means you have to select an inverter which is big enough to supply this increased load for a short time (ie it has a high "surge" rating). I don't have any actual surge figures for real fridges unfortunately.

 

I presume you already have the fridge? If you are getting a new fridge, and an inverter just to power it, a 12 volt fridge may be a better option. The fridges are expensive but they are (according to the manufacturers' figures) more efficient and you save the cost of the inverter

 

Allan

[Theo]

I think it is most likely to be a problem of potential damage to the inverter, or else the system not working because of the inverter shutting down to protect itself. I would imagine that simply using a larger, pure sine wave inverter would get you round the problem.

 

Another reason that many inverters have in running a fridge, is that most 240v fridges have quite a high startup current every time the motor kicks in. It's not a big problem when they are installed in a house, but on a boat it means you have to select an inverter which is big enough to supply this increased load for a short time (ie it has a high "surge" rating). I don't have any actual surge figures for real fridges unfortunately.

 

I presume you already have the fridge? If you are getting a new fridge, and an inverter just to power it, a 12 volt fridge may be a better option. The fridges are expensive but they are (according to the manufacturers' figures) more efficient and you save the cost of the inverter

 

Allan

 

 

I have neither at the moment but 230V fridges are cheap and I will need to get an inverter anyway so I thought that this would be the cheapest way.

 

Nick

[NB Alnwick]

I have neither at the moment but 230V fridges are cheap and I will need to get an inverter anyway so I thought that this would be the cheapest way.

 

If you have gas then the three-way fridge works out cheaper in the long run even though it is a lot more expensive to buy. Electric fridges will be a tremendous drain on your batteries but we can leave our fridge running on gas for weeks with all the power switched off and it only uses a very small amount of gas.

 

Here is a quick link to the Electrolux range of gas/dc/ac fridges:

 

http://www.obrienscamping.co.uk/FridgeRange.htm

Edited January 11, 2007 by NB Alnwick

[Keeping Up]

If I remember correctly the old Electrolux 3-way fridges used to have no thermostat when they ran off 12 volts, they just took the maximum power all the time and were very inefficient. They were fairly inefficient when running off 240v too, but were cheap to run on gas - although I believe it can be quite difficult to fit them in a way which complies fully with the BSS regulations (I believe they can be fitted without being room-sealed but they must vent their exhaust to the outside, is this correct?).

 

A year ago I compared the published power consumption figures for modern electric fridges. If the figures are accurate, for the same medium size fridge, an AA rated 240 volt one uses nearly twice as much power as a good 12 volt one. On a 12-volt system this is an extra 20 to 25 Amp-hours per day. The extra power requirement may mean you have to run your engine for longer (for about half an hour longer every day if you can charge at 50 Amps), or even add an extra battery.

 

All the same if you want to have a decent sized inverter anyway, then running a fridge from it may well be the best option for you. In that case, get one with plenty of spare power in hand to cope with the problems you describe in your original post, and with as high a surge rating as possible - better still, find a combination of fridge and inverter which you know will work because either the inverter supplier says it will or else you have spoken to someone who has got that combination and says it is OK.

 

Allan

[David Schweizer]

Several years ago one of the Canal Magazines did a comparative test between modern 12v DC fridges and 240v AC fridges run from an inverter. They came to the conclusion that capital outlay was slightly cheaper for 240 volt AC plus inverter, but that the 240v/inverter approach took more from the batteries.

 

The other thing they identified was that because fridges need a lot of initial power to start the compressor, you needed an inverter with an output which is a least twice that needed to run the fridge, I think that is what is meant by the inductuve load, but there will be people here who will probably be able to explain this more clearly.

[NB Alnwick]

If I remember correctly the old Electrolux 3-way fridges used to have no thermostat when they ran off 12 volts, they just took the maximum power all the time and were very inefficient. They were fairly inefficient when running off 240v too, but were cheap to run on gas - although I believe it can be quite difficult to fit them in a way which complies fully with the BSS regulations (I believe they can be fitted without being room-sealed but they must vent their exhaust to the outside, is this correct?).

 

The 3-way fridges have moved on a tad - ours has thermostatic control on all three modes and the latest versions are fully automatic:

 

AES - Automatic Energy Selection

 

This automatic really pays off. You switch it on once, that's it. The advanced AES system automatically selects the best energy source, in the hierarchy 230 Volt, 12 Volt and gas.

 

When you start the fridge for the first time you set the cooling temperature you desire. The system will maintain this temperature, regardless of the power source or the outside temperature. Whether you are moving or stationary, the best energy source is always used. When the engine is turned off, the system waits 15 minutes before switching from 12 Volt to gas. This delay ensures that gas operation is not possible when you stop at a petrol station. The system measures the voltage continuously (even in mains stationary mode, which is very useful not only in Mediterranean countries) and switches to gas if it detects a voltage drop to ensure sufficient cooling performance.

 

If Nick is buying a fridge anyway - the new 3-way versions a well worth a look - if only for the fact that you can leave the boat unattended with the fridge running and all the electrics switched off for weeks on end - we do.

 

We only speak as very satisfied users and have no connection with the producer or retailers - sadly nor do we have any electrical expertise so that must be left to the experts on this forum.

[Gibbo]

Since it looks as if I am going to buy a new inverter I have been studying the data on the internet. Just one of the modified sine wave inverters that I looked at said that it would not run a fridge because of the problems with an inductive load. The other MSW units that I looked at did not mention any problems in that way. I wonder if it is a problem of damage to the fridge motor or damage to the inverter. Can I overcome the problem, if it exists, by derating?

 

I would be interested to hear your views.

 

Nick.

 

I have a *lot* of experience with fridges and inverters.

 

I've never known a 230 volt fridge that wouldn't run from a MSW inverter because of the waveform. Obviously I've come across some problems with insufficient power. A 1500 watt inverter will run a normal size domestic fridge. *Some* 1000 watt inverters will run one. And I've come across some 600 watt inverters that will run them, but that's getting a bit hit and miss.

 

I know of several hundred fridges that ran successfully from MSW inverters.

 

There have been some problems with fridges running from inverters that have a power save/idle function. The fridge won't try to fire up until it has 230 volts available, the inverter won't produce 230 volts until it sees a load, catch 22 (there is a cure).

 

About 10 years ago (perhaps a few more) I ran a load of tests on power consumption comparing 12 volt fridges to 230 volt units running through an inverter. At that time there was very little difference between them. Certainly not enough to justify paying 4 times the price for a 12 volt fridge if an inverter was already there.

 

I did similar tests last year. Both types had improved *dramatically* in efficiency, but the 12 volt units had improved far more (don't ask me why, I have no idea). Modern fridges of the 12 volt variety now use about half the power that a comparable 230 volt unit uses. But they cost 4 times as much.

 

Hope that helps.

 

Gibbo

[NB Alnwick]

About 10 years ago (perhaps a few more) I ran a load of tests on power consumption comparing 12 volt fridges to 230 volt units running through an inverter. At that time there was very little difference between them. Certainly not enough to justify paying 4 times the price for a 12 volt fridge if an inverter was already there.

 

I did similar tests last year. Both types had improved *dramatically* in efficiency, but the 12 volt units had improved far more (don't ask me why, I have no idea). Modern fridges of the 12 volt variety now use about half the power that a comparable 230 volt unit uses. But they cost 4 times as much.

 

We reckon the extra cost of a 3-way gas/dc/mains fridge is justified on the grounds of flexibilty but it would be interesting to know just how many weeks a fridge would run if left in a remote mooring before it flattened the batteries? as an example we have four x cabin batteries rated at 95 amp/hour.

[Gibbo]

We reckon the extra cost of a 3-way gas/dc/mains fridge is justified on the grounds of flexibilty but it would be interesting to know just how many weeks a fridge would run if left in a remote mooring before it flattened the batteries? as an example we have four x cabin batteries rated at 95 amp/hour.

 

Reckon on an average of about 30 to 40 amp hours per day for a modern 12 volt fridge (obviously it varies drastically depending upon installation, useage, ventilation etc) so not very long!

 

On most narrowboats it's the biggest user of electricity. Not on mine as I have 2 kids with x-boxes, playstations, 3 TVs, 2 DVD players etc etc etc

 

I have an aversion to gas fridges on CO grounds that I refuse to get into. At all. Ever again. But I wouldn't have one given me.

 

Gibbo

[NB Alnwick]

Reckon on an average of about 30 to 40 amp hours per day for a modern 12 volt fridge (obviously it varies drastically depending upon installation, useage, ventilation etc) so not very long!

 

So it would completely flatten our batteries in about eight days? That is why we go with gas - replacing a gas bottle is cheaper than replacing batteries - but we are not saying that other people should change their ideas - we all have different needs . . .

[DHutch]

If I remember correctly the old Electrolux 3-way fridges... .were fairly inefficient when running off [12v and] 240v too, but were cheap to run on gas...

Yes. Well "three way" or "gas" fridges work on an absorbtion refriegation cycal, rather than a compressor based cycal, which pritty much inhearently makes them grossly inffecient in comparisen. And when there running of electricity, there even worse, becuase the only way they do that is to have a heating element where the flame would be.

 

I personaly strongly favour a compressor-based fridge, for the inherently safer and simpler running. And certainly its very compatable with touring/cruising. It works very well on emilyanne, even with a 15yo fridge, and a rather limited alterntor output.

 

However, i could see that for a liveaboard with no shoreline, the gas fridge route is still a very attactive option.

 

 

Daniel

[alan_fincher]

The 3-way fridges have moved on a tad - ours has thermostatic control on all three modes and the latest versions are fully automatic:

If Nick is buying a fridge anyway - the new 3-way versions a well worth a look - if only for the fact that you can leave the boat unattended with the fridge running and all the electrics switched off for weeks on end - we do.

 

 

We reckon the extra cost of a 3-way gas/dc/mains fridge is justified on the grounds of flexibilty but it would be interesting to know just how many weeks a fridge would run if left in a remote mooring before it flattened the batteries? as an example we have four x cabin batteries rated at 95 amp/hour.

 

I'm not sure it would run on batteries for very long.

 

Someone help me out here, because at first I thought my maths has to be wrong, (but now I'm not so sure!)....

 

Even the very small 40 litre Dometic 3-way fridge shown in the link you posted says it uses around 2.2 KW/hr of electric in a 24 hour period. (It doesnt say if that's on 12v or 240 volt, but to progress lets assume either is similar).

 

So I want to say that if you are using 2,200 Watt hours in 24 hours, then from a 12volt supply that is 2,200 / 12, or about 183 Amp hours.

 

If you have 4 * 95 Amp hour batteries, (i.e. a nominal 360 Amp hours total), and even if you you assume 100% efficiency, but follow a guideline of never more than half discharging them, then to me it sounds like that's only about a days use of the fridge !

 

(I don't want to enter an argument about what you can or should actually take out of the batteries - I'm just being VERY approximate here).

 

I'm happy to be told I've got it wrong, but if I haven't it still sounds like even the most modern 3 way fridges are highly inefficient on 12 volts, compared to a compressor based type.

 

It's been said in the past that their intended 12 volt use is only for when they are in a camper van or caravan on the move, with an engine running, so battery drain is far less relevant. It doesn't sound to be much different now, unless my numbers are wrong.

 

Incidentally, in the past Electrolux sold these fridges as for caravan type use, and would never I believe actually support their use in a marine environment, (although of course thousands have been so used over the years). I'd be interested to know if that changed now they are Dometic ?

 

Finally, for Allan, so far as I recall, in the latest BSS guide, you are not even compelled to arrange that a fridge running on gas is flued to the outside world. I'm pretty sure it can just burn away in the cabin, with no more than the natural ventilation of the boat, (not that I'd do it mind!). If I'm correct, that was quite a change from the draconian restrictions in earlier versions of the regs. (Again I'm quite prepared to be told I'm wrong, though!).

 

EDIT,

 

Just cross posted with Gibbo and Alnwick.

 

Obviously Gibbo's usage numbers for a 12 volt fridge are a fraction of what I'm suggesting here. I don't believe that's inconsistent, if he is talking about a modern 12 volt compressor based unit, whereas I'm assuming the Dometic offering is still basically just using an electrical heating coil to replicate what it would be doing if running on gas....

 

(Once more though), I'm still quite prepared to learn that a modern Dometic is in some way a different beast from an old Electrolux model...

Edited January 12, 2007 by alan_fincher

[NB Alnwick]

It's been said in the past that their intended 12 volt use is only for when they are in a camper van or caravan on the move, with an engine running, so battery drain is far less relevant. It doesn't sound to be much different now, unless my numbers are wrong.

 

Incidentally, in the past Electrolux sold these fridges as for caravan type use, and would never I believe actually support their use in a marine environment, (although of course thousands have been so used over the years). I'd be interested to know if that changed now they are Dometic ?

 

Finally, for Allan, so far as I recall, in the latest BSS guide, you are not even compelled to arrange that a fridge running on gas is flued to the outside world. I'm pretty sure it can just burn away in the cabin, with no more than the natural ventilation of the boat, (not that I'd do it mind!). If I'm correct, that was quite a change from the draconian restrictions in earlier versions of the regs. (Again I'm quite prepared to be told I'm wrong, though!).

 

Thanks for the calculations - I am sure that we made the right choice with our 3-way fridge - it should be running now keeping our cheeses, butter and puddings nice and cool - even though we have been away from the boat for more than a week.

 

It was professionally installed and it does vent and exhaust to the outside.

 

The new ones that automatically select the most efficient power source seem to be very good indeed - especially since we often forget to turn the manual switch and just leave it on gas even when we have the engine running.

 

The other advantage is that they are silent in operation.

 

 

When we last went out in a hired boat the fridge was quite noisy and made the whole boat shudder when the motor kicked in - out on a quiet mooring in the countryside, it could be quite annoying especially at night.

[Gibbo]

I'm not sure it would run on batteries for very long.

 

Someone help me out here, because at first I thought my maths has to be wrong, (but now I'm not so sure!)....

 

Your maths is right.

 

 

Obviously Gibbo's usage numbers for a 12 volt fridge are a fraction of what I'm suggesting here. I don't believe that's inconsistent, if he is talking about a modern 12 volt compressor based unit, whereas I'm assuming the Dometic offering is still basically just using an electrical heating coil to replicate what it would be doing if running on gas....

 

Which it is.

 

 

(Once more though), I'm still quite prepared to learn that a modern Dometic is in some way a different beast from an old Electrolux model...

 

'fraid not. It always seemed ironic to me that a fridge made by ELECTrolux was only any use when running from gas.

 

Gibbo

[alan_fincher]

Don't get me wrong, Graham.

 

I'm not knocking them, just that I wouldn't want to try running it on 12 volts for too long, unless on the move.

 

We actually have a fairly elderly Electrolux 3-way, but with no on-board 240 volt, and quite modest 12 volt batteies and charging arrangements, it's actually not wired in to anything.

 

We ONLY run on gas, therefore. Personally it doesn't bother me, as that's what I had years ago, (as most boats did then, I think), and despite the scaremongers, I've not seen much to support any carbon monoxide narrowboat deaths relating to fridges.

 

Heating appliances seem to present the greater risks, from what I've seen.

[DHutch]

Reckon on an average of about 30 to 40 amp hours per day for a modern 12 volt fridge (obviously it varies drastically depending upon installation, useage, ventilation etc) so not very long!

- On most narrowboats it's the biggest user of electricity.

Is that a compressor fridge, or a 3-way?

 

Eatherway. Our old Engal uses about 50watts when running (2amps at 34v)

- So say its runing 50% of the time (quite accuratly, as it usally runs non-stop 8till8, then off overnight)

- Thats a totat of 50*12watthours a day, or 50amphours a day at 12v.

 

The newer compresor fridges (shorelines etc) are as gibbo says, quite a lot better, i think around 30/40 Ah per day (what sort of unit is THAT!!)

- And without having looked them up recently, i thought the consumption for the 3-ways was consderatbly more than that, say 80/100AH a day off the top of my head.

 

So yeah, on our 2*2*110Ah bank, using the magic 50%, about four half days with the engle, a good 6days with a shoreline, and around 2 or so with a 3-way.

 

- Which actually sounds about right for what we find, we can do three days without power, with slightly frugal fridge/light usage. but then its about time to get out the genny...

 

 

Daniel

[Gibbo]

Is that a compressor fridge, or a 3-way?

 

Compressor. For a three way treble it, or perhaps even more.

 

The newer compresor fridges (shorelines etc) are as gibbo says, quite a lot better, i think around 30/40 Ah per day (what sort of unit is THAT!!)

 

It looks odd you're right, but it's perfectly legit. Ahrs aren't amps *per* hour, they are amphours.

 

Gibbo

[chris w]

The 12v fridge on my new boat is reckoned to be about 4 or 5 amps @ around 50% duty cycle. (ie: on half the time).

 

With your 4 x 95AH batteries (assuming they are rated at the 20 hour rate) the actual capacity (taking the Peukert factor into account) is 919AH of which we can use half before they ought to be recharged, say 450AH. The Peukert equivalent of 5A is 8A (5^1.3), so dividing this into 450AH gives 56 hours of use. If the duty cycle is 50% for the fridge this 56 hours will take 112 hours (or 5 days) to use up.

 

So, it looks like a working week before the batteries need recharging for the system above. If the current were 4A rather than the 5A I used in the calculation above, this would add an extra day to the discharge time (ie: 6 days instead of 5). At 3A it increase to 9 days due to the exponential effect of the Peukert factor.

 

Chris

Edited January 12, 2007 by chris w

[WJM]

I think you should look at an inverter as a nice little extra - but never allow it to be an essential. All your essential equipment should idealy run off the good old reliable 12v supply. An inverter is just another thing to go wrong - and it makes an annoying buzz, so I always turn mine off at night.

[Yamanx]

I run a normal 230v fridge I bought for 20 quid off Ebay from a msw inverter without any issues. Its a sterling 2500 watt, and incidently doesnt buzz, the only sound it makes is the fan when its hot, which doesnt happen at night.

 

Because I dont live aboard and only use the boat for leisure, there's not more than a day, at the most two when the boat isnt moving, which keeps the batteries charged. Of course I've only been using the boat during the winter so far, so in the summer when its warmer (Although not much eh?) there may be an issue with the fridge running down the batteries, we'll see.

[magnetman]

I think you should look at an inverter as a nice little extra - but never allow it to be an essential. All your essential equipment should idealy run off the good old reliable 12v supply. An inverter is just another thing to go wrong - and it makes an annoying buzz, so I always turn mine off at night.

 

I agree with this - they seem to be less than entirely reliable (maybe you've got to spend £1,000 to get a good one).

 

My 12v Danfoss compressor fridge works well, specially in the summer with a 75w solar panel to help out, keeps the beers well chilled on those hot days.

 

I met a bloke on the Fens who said it is possible to replace the thermostat (or controller or something) with a microprocessor unit which increases the efficiency of the fridge in some way.

[blackrose]

So it would completely flatten our batteries in about eight days? That is why we go with gas - replacing a gas bottle is cheaper than replacing batteries - but we are not saying that other people should change their ideas - we all have different needs . . .

 

I'm with you on this one Alnwick. I have an instinctive dislike of anything that's going to constantly put a load on my batteries. I've seen too many boaters spending time trying to keep up with charging batteries that are under constant load & problems of drained domestic battery banks, often due to 12v or mains fridges on inverters. I don't have a huge domestic battery bank (405A/h) and use it to supply intermittent loads like water and shower pumps as well as 12v lights, car stereo and TV from the inverter, all of which can be switched off if the batteries are getting low. What are you going to do at midnight before bed when you realise that you hadn't charged enough that day & your fridge is starting to kill your batteries? Start your engine? Your neighbours are going to love you in the morning!

 

I had an old Electrolux 3-way fridge on my previous narrowboat and it was good. A 13kg gas bottle used to last me about 6 weeks without the fridge running and about 4 weeks with the fridge on. It just burns a small pilot light. On this boat I bought a Thermocool 3-way fridge. I'd never heard of the brand but it was cheap and the build quality looks similar to any of the other 3 way fridges I've seen. I'm only using it on 240v mains at the moment as I haven't installed gas yet. These fridges are tiny compared to mains appliances, but I only need a few things in there and I don't want to power a big household unit when I'm off the mooring.

Edited January 12, 2007 by blackrose

[alan_fincher]

The 12v fridge on my new boat is reckoned to be about 4 or 5 amps @ around 50% duty cycle. (ie: on half the time).

 

With your 4 x 95AH batteries (assuming they are rated at the 20 hour rate) the actual capacity (taking the Peukert factor into account) is 919AH of which we can use half before they ought to be recharged, say 450AH. The Peukert equivalent of 5A is 8A (5^1.3), so dividing this into 450AH gives 56 hours of use. If the duty cycle is 50% for the fridge this 56 hours will take 112 hours (or 5 days) to use up.

 

So, it looks like a working week before the batteries need recharging for the system above. If the current were 4A rather than the 5A I used in the calculation above, this would add an extra day to the discharge time (ie: 6 days instead of 5). At 3A it increase to 9 days due to the exponential effect of the Peukert factor.

 

Chris

But if it is a Dometic 3-way fridge, most will, I think, take a far larger current than this....

 

Even the smallest model on their web-site, (and I'm guessing that Graham's will be larger), quotes 100 watts, (so around 8 Amps consumption, rounded to nearest whole Amp).

 

Also the model who's spec I have just looked at says "Thermostatic control on mains operation ", which I take to mean (like the old Electrolux models) "NO thermostatic control on 12 volt operation".

 

So I think (depending on model) a small basic Dometic 3-way could well end up drawing 8 amps continuously, rather than 4 or 5 amps only 50% of the time.

 

I don't want to dive into "peukert equivalents", (your speciality, not mine!), but I'm suspecting if you redo your more complex maths for a basic 3-way model, that the answer might well come out very roughly around the one days use I was suggesting.

 

However Graham reckons his has a thermostat on 12 volts, so he presumably would get a run time somewhere between my number and your number. If he can tell us the fridge model number, I guess it would be possible to look at the exact spec, and give a better prediction of when he might run out of battery ?